A downlink (DL) multi-user (MU) transmission method in a wireless local area network (WLAN) system, the DL MU transmission method including receiving a DL MU physical protocol data unit (PPDU) including a physical preamble and a data field from STA (Station) and transmitting the ACK frames in response to the DL MU PPDU to STA. In addition, the data field includes at least one medium access control (MAC) protocol data unit (MPDU), the at least one MPDU includes a MAC header and a MAC frame body, wherein the MAC header includes acknowledge (ACK) indication information, the ACK indication information includes frequency resource allocation information for an uplink (UL) MU orthogonal frequency division multiple access (OFDMA) transmission of ACK frames and modulation and coding scheme (MCS) level information, and the frequency resource allocation information includes an index value indicating a resource unit allocated for the UL MU OFDMA transmission of the ACK frames, and the resource unit corresponds to a 26-tone resource unit, a 52-tone resource unit, a 106-tone resource unit, a 242-tone resource unit, a 484-tone resource unit, or a 996-tone resource unit.